Machines such as excavators, dozers, loaders, motor graders, and other types of heavy equipment use one or more hydraulic actuators to move a work tool. These actuators are fluidly connected to a pump on the machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuator and the connected work tool. In an open-loop hydraulic system, fluid discharged from the actuator is directed into a low-pressure sump, from which the pump draws fluid. In a closed-loop hydraulic system, fluid discharged from the actuator is directed back into the pump and immediately recirculated.
Regeneration within an open-loop system may help to increase an efficiency and/or speed of the system. Regeneration during extension of a hydraulic cylinder is typically accomplished by connecting a rod-end chamber of a hydraulic actuator directly with a head-end chamber of the same actuator, while also supplying fluid from the pump to the head-end chamber. As the pressure within both chambers during regeneration may be about equal, the hydraulic cylinder will extend due to an imbalance of forces created by the pressure acting on disproportionate areas within the two chambers. Because the head-end of the hydraulic cylinder is being supplied with fluid both from the pump and from the rod-end chamber during extension regeneration, the hydraulic cylinder may be able to move faster and/or have fewer losses than otherwise possible.
Regeneration within a closed-loop system has historically not been as effective as within the open-loop system described above. In particular, when the rod-end of a hydraulic cylinder is directly connected to the head-end of the same cylinder, the closed-loop system may be pressure-limited by associated charge relief valves that are generally required within a closed-loop system. Although high-pressures may not be necessary during extension regeneration, an open-loop system operating at higher pressures will generally outperform a closed-loop system operating at lower pressures.
An exemplary closed-loop system having enhanced regeneration is disclosed in Japanese Patent 2011/069432 of Takashi et al. that published on Apr. 7, 2011 (the '432 patent). The '432 patent describes an over-center, variable displacement pump connected to a hydraulic cylinder. During normal operation, the pump is connected to the hydraulic cylinder in closed-loop manner. However, during regeneration, the pump is connected to only one chamber of the hydraulic cylinder in an open-loop manner. An accumulator is utilized to selectively store high-pressure fluid discharged from the hydraulic cylinder during regeneration and to selectively supply fluid to the pump during normal operation. A charge circuit provides makeup fluid to the pump during open-loop operation.
Although an improvement over conventional hydraulic systems that have a permanent closed-loop configuration, the system of the '432 patent described above may still be less than optimal. In particular, the system of the '432 patent may be overly complex, expensive, and difficult to control. For example, the system of the '432 patent may include a great number of different types of valves that control complicated fluid flows throughout the system. These valves, along with the associated fluid flows, increase an overall cost of the system, while simultaneously increasing computing and control requirements.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art.